All you ever wanted to know about Mycelium

by

Dr Juliet C Frankland - reported by Rita Cook

Published in the April 1997 issue of NWFG Newsletter (ISSN 1465-8054)

Introduction

The talk given at the A.G.M. by Dr Juliet Frankland, our President this year, had a new focus. It was essentially a fascinating in-depth study of one small fungus and its interaction with the environment, rather than a wide-ranging review of the many species growing in a particular sphere. As such its richnesses were perhaps a little more difficult to absorb at one sitting.

However Juliet was kind enough to let me have the text of her talk with permission to summarise and edit it as I thought fit for inclusion in this issue of the Newsletter.

You may now read and inwardly digest the interesting material at your leisure!

For the benefit of the many beginners among us I have attempted on occasion to simplify the scientific terminology used for I feel it is important to try to communicate the excitement of scientific research to the lay person and this really includes all of us in the Group.

I hope that Juliet will understand this and forgive me for any errors in interpretation I may have made. It has all been done with the best of intentions (with which Hell is paved!).

All about mycelium

The general theme of the talk was the importance of mycelium, its whereabouts and interactions and in this context Juliet described the research she had done into the mycelium of one particular decomposer fungus which is very familiar to all of us - Mycena galopus.

This species is one of that group of fungi which act as refuse disposal and recycling agents, a modest, not very spectacular "little brown job" which oozes white milk from the stipe when it is broken.

The beginners among our members might like to be reminded that the mycelium is the most important, permanent part of the fungus. Initially it springs from fallen spores in the form of tiny threads (hyphae) which ultimately spread and multiply to form a mat or network which permeates the soil or other substrates, in search of nutrients. It is rather like the branches of an upside-down "tree" growing below ground whose "fruits" are the familiar mushrooms and toadstools we see emerging from it above ground.

Since, like the fruits of plants, these fungi are temporary structures and merely the more visible part of the reproductive system, it is obviously less damaging to collect them than to remove or destroy the less visible mycelium, the "business bit" of the fungus which though hidden from sight continues to perform the major function of decomposition and cycling of nutrients even when conditions are unsuitable for the production of fruit-bodies for perhaps several years.

America (though not Texas!) - where else?

The largest, possibly the heaviest, and maybe the oldest organism alive was at first thought to be a mycelium found in the forests of Michigan, N. America. This giant is the mycelium of the parasitic fungus, Armillaria bulbosa (a form of Honey Fungus), which has sprung from a single spore and grows mainly underground.

It is now estimated to cover a minimum 40 acres (150,000 m²), making it 16.8 times the size of the pitch and surrounding grass of Wembley Stadium. Scientists have calculated its weight to be over 100 tonnes and it is at least 1,500 years old.

But since many different species of fungi with their own mycelia grow together in close proximity in any forest, how can one be certain that this giant is just one fungal mycelium and not a mass of several springing from several different spores falling from several different fruit-bodies all living in the same soil?

The answer is that we can now apply the latest genetic fingerprinting techniques to fungi (as we saw happening at David Conways' table at the Fungus 100 exhibition reviewed in our editorial). Sections of the DNA of any fungus may be taken from various sites and compared. If the same pattern is found in two samples it is virtually certain that they are from the same organism.

The tests made in Michigan proved that all the samples taken over the 40 acres were genetically identical. However the Michigan fungus's reign as official Largest Organism in the World was short-lived. Hot on the heels of the announcement of their monster find came another from Washington State. This time the fungus in question was Armillaria ostoyae.

For about 20 years it had been suspected that the mycelium was an individual. And now, "When we heard about Michigan" said Forest Health Department Manager, Ken Russell, "we said, we can't have the braggin' rights residing in Michigan when weve got a bigger fungus". They claimed that this relative of the Michigan fungus covered a mind-boggling 1500 acres and was between 400 and 1000 years old. However it was suspected in some quarters that Ken Russell might be pre-occupied with "braggin' rights" at the expense of scientific accuracy.

Nevertheless, the Journal Nature was alerted to the existence of the Washington find and published a leading article entitled "The Great Fungus" which effectively fired the starting-gun on a search for an even bigger one. Fungus hunters were now said to be scouring Oregon.

Beatrix Potter - sexism raises its ugly head!

As an introduction to her talk Juliet reminded us that Beatrix Potter of Peter Rabbit fame had presented a paper about the germination of spores 100 years ago to the prestigious Linnean Society of London, but being a woman had not been allowed to read the paper or even be in the room!

Even at that date she was intrigued to know the origin of the fruit bodies she painted so beautifully and recognised that the mycelium in the soil was of great significance. She produced very accurate drawings of the hyphae (threads forming the mycelium) depicting clamp connections (bumps or knobs) on them, features involved in the reproductive process but not explained at that date.

Sadly her observations were dismissed as unimportant and the paper lost. At the time, she wrote in her encoded diary: "It is odious to a shy person to be snubbed as conceited, especially when the shy person happens to be right". As a result she abandoned her scientific career and concentrated on childrens books.

However some of her scientific drawings have been preserved by the Armitt Trust and may still be seen in the Armitt Library at Ambleside which some of us hope to visit in May. And, in April, Professor Roy Watling, principal mycologist at the Royal Botanic Garden, Edinburgh, is to deliver a lecture on her work to the Linnean Society and the overdue apology may be made.

Juliet's Talk

The title of Juliet's talk was: "Fungal Succession - Unravelling the Unpredictable". She explained succession as "the sequential occupation of the same site by mycelia either of different fungi or of different associations of fungi". In other words as the environment changes for various reasons, so different fungi "take over" possession of a site by invading it with the tiny threads (hyphae) which form the mycelium (mat of threads).

We cannot, in the space of this article, reproduce the whole of the wealth of information she presented us with. Instead we shall focus on her account of the interesting experiments she carried out with Mycena galopus, for these illustrate many of the theoretical principles she describes.

While a complete "take-over bid" for almost the whole of a forest seems to have been achieved by the giant fungi of Michigan and Washington State it is important to point out that any one suitable substrate or medium can be colonised by a series of fungi that replace each other as the various nutrient components are used up and as conditions change. Many fungal successions have been found and described on decaying plant remains. Juliet herself has spent much time examining microfungi (those invisible to the naked eye which tend to appear first) on leaf litter of common trees, including oak, birch, hazel and ash at Meathop Wood near Grange-over-Sands.

But the study of these organisms revealed little of how one species replaced another. And in early accounts researchers tended to concentrate on microfungi. Very few mycologists recorded basidiomycetes, i.e. the larger fruit-bodies with which we are so familiar and which are obvious to the naked eye. A reminder to beginners again that basidiomycetes (=fungi with basidia) are so-called because they carry their spores on tiny "clubs" (basidia) which can be seen, though only with the aid of a microscope, in the case of many of the larger fungi, on the gills. These larger fungi were to give some of the answers sought. Six months after the initial tests at Meathop Wood, another laboratory experiment took place. This time a small basidiomycete, Mycena galopus, appeared in great numbers when leaves showing signs of white rot were incubated in damp chambers for several weeks. The fruit bodies were not formed properly, but the white milk in the stipes confirmed the identification and 80% of the leaves were found to have been colonised by this species. This discovery triggered a series of experiments which explored the behaviour of Mycena galopus in its natural environment. How and why does this little fungus appear and replace other fungi?

Before describing the experiments it might be useful to include a little general theory by asking some questions.

How do fungi arrive at their destination?

Replacing other species is the crux of succession but how does the fungus arrive at its destination in the first place and so overcome and succeed other fungi? How does it get there? Both adequate space and a supply of spores and/or mycelium are needed before it can establish itself. Early mycologists concentrated on arrival as spores. These are continuously floating around in the air having been dispersed through various mechanisms by the fruit-bodies whose function it is to produce them. And, though we cannot see them with the naked eye, they are the more readily measurable of the two agents. But more recently the focus has been on examining the nature and spread of mycelium at all stages. Great progress has been made by direct observation of the behaviour of the mycelium which allows fungi to establish themselves.

More about mycelium

The mycelium of any fungus is very dynamic and adaptable.

1. It may lie dormant in a substrate for long periods until conditions are right for its further development.

2. In addition some sort of homing mechanisms have been discovered whereby a mycelium may grow towards a particular nutrient source such as a root.

3. It may also change shape, from a compact fluffy state to thin compressed growth or well-defined cords (as with Honey Fungus) that can explore through a forest for great distances.

4. Other features also occur when two mycelia meet. a) they may just intermingle, or b) sit there in deadlock, or c) "combat" may take place and one overgrow the other and replace it. A model for the science fiction writer!

Mycena galopus - at last!

With the above especially applicable factors in mind we can now look at the long-term study of Mycena galopus which Juliet undertook. Some general facts first. This little fungus is common in northern temperate regions of the world. Though not very spectacular, as we said previously, it is easy to identify by the white milk or latex in its stipe (stem).

In many deciduous and coniferous woodlands it is a major litter decomposer. It occurs on oak leaf and bracken successions - on oak it has peaked 2 years after leaf fall, usually one mycelium per leaf, sometimes two with a black "warfare" zone between them. In Grizedale Forest (see below) with a continuous bed of Sitka spruce needles it occurs in large troops. It will also grow on other plants including moss.

It was tested in the laboratory and found to attack all the main components of plant tissues but was particularly vigorous at attacking lignin and cellulose, producing patches of white rot. It is active over long periods even at low temperatures under snow.

The aim now was to discover the whys and wherefores of its occurrence in some Lake District woodlands, and to investigate the location of its mycelium as a secondary colonist in successions on plant litter.

Arrival

Its arrival in the first site studied, Grizedale forest, was investigated initially by capturing its spores from the air. There can be no mycelium without spores.

Tests proved that after a period the hyphae (threads) produced by these air-borne spores mated successfully with the hyphae of other "ready prepared spores" which had been purposely exposed in the forest. Mating was confirmed by the appearance on the hyphae of the hitherto mysterious clamp connections ("bumps") which appeared in Beatrix Potters drawings 100 years ago. This mating is the essential process by which the mycelium develops. So all the evidence pointed to their being plenty of viable spores in the forest.

Space

Once the spores had germinated, mated and started to produce mycelium was there space for the colonies to become established and did they persist? The mycelium is nondescript, white and like that of many other species so cannot be used as an indicator. However the fruit bodies do provide a clue. They were mapped in permanent quadrats every few days through a season and tissue was taken from the stipes to determine their compatibility with each other and whether they belonged to the same mycelium. They did and in this way perennial mycelia were discovered, the largest about 2.5 metres in diameter.

Depth

Another experiment indicated the depth at which the mycelium was located. By excavating the fruit-bodies to the point of origin on the mycelium in the soil, the mycelium was found to be just below the surface layer of needles where the needles were black and partly decomposed.

Competition

Was there competition from similar species? Marasmius androsaceus, the little Horsehair Fungus, often grows in mixed clumps with the Mycena in Grizedale. When they were grown in the lab. together, the Marasmius always out-competed the Mycena, growing faster and decomposing more litter. So how did they fare in the field? Again, the mycelium could be located by tracing fruit-bodies down to their source. It was found that when Marasmius and Mycena grew together in this environment Marasmius out-competed the Mycena at the highest level but not at the lowest. At the surface, the Mycena mycelium was displaced downwards and competed better there. There was in fact a vertical zonation of the mycelium of both species and Marasmius was potentially the better competitor.

Why then did the Mycena predominate at the greater depth. The answer lay with a small animal, a Springtail, which is a fungal feeder but much prefers eating the Marasmius mycelium. Student Katy Newell suggested that Marasmius with its rhizomorphs (thick strands of compacted hyphae) could survive droughts on the surface litter. Springtails do not like dry conditions and so would not eat the surface mycelia. Consequently the Marasmius flourished at this level. In dry conditions the Springtails would retreat deeper into the litter and attack the Marasmius mycelium there. This weakened it, leaving the way clear for stronger development by the Mycena mycelium which was relatively unpalatable to the small creature. Several field experiments supported this hypothesis.

When 5-year records of the positions of M. galopus fruit-bodies in Grizedale were computer-mapped, it was found that they do not occur randomly and that the mycelia were perennial or at least renewable year after year at the same locations. The mapping also showed that rings or arcs of fruit-bodies occurred around the Sitka spruce trees.

In good fruiting years these could be seen without repeated mapping i.e. on a single visit. It was also found that there was a relationship between the size of the tree trunks and distance of the rings from the tree; the bigger the trunk, the further out the ring. This was unexpected. What were these rings?

M. galopus appeared to be behaving like a mycorrhizal fungus (the fungi which have a symbiotic relationship with trees via their root systems) but it was not mycorrhizal. Was this, then, a "fairy ring?" Again, not possible, for tests showed that the M.galopus mycelium was present all the way from the trunk to the ring whereas in ordinary "fairy rings" the mycelium has degenerated in the centre. Nor could the position of the fruit- bodies at a particular distance from the trees be explained, though many reasons have been suggested. Concentric patterns of various soil properties are well-known to occur around trees and that might be responsible. The arched root systems of spruce could well be draining surface water to the perimeter of each tree site. A significant increase in moisture content of the litter outwards from the tree was been measured but did not peak at the rings. Ammonium nitrogen is significantly greater at the ring position but the source is not known; it may come from the fruit-bodies themselves as they decompose. Depth of litter also appears to be greatest at this point but more data are required. The quest for answers goes on.

Conclusion

So there has been a slight unravelling of the ecology of one species in one locality only. Its arrival and establishment have been investigated. What about its replacement as its resources are exhausted? Lower down, in the humus layer, soil moulds have replaced it. But, although it has been replaced, this fungus is obviously a major factor in maintaining the ecosystems where it occurs in abundance, with far more mycelium below ground than above, recycling the very nutrients vital to tree growth.

Postscript

The Greater Manchester sub-group has "adopted" a site near Macclesfield. This is Lyme Green Park, a new 15 acre private nature reserve where planting has recently begun. Its owner claims it will be the largest deciduous woodland in Macclesfield and the largest new deciduous woodland site in the North West. Although we cannot mount an ambitious project of the type just described, we hope to visit the site frequently in order to record the succession of fungi as they appear throughout a period of several years.